Ablation is the removal of material from the surface of an object by vaporization, chipping, or other erosive processes. The term “ablation” is often used in the context of laser ablation (i.e., laser machining), a process in which a laser dissolves bonds in a solid or sometimes liquid material. As a result, small fragments of the material in the form of gases, small liquid and/or solid droplets or particles are freed from the material and either carried away by a gas stream or re-deposited on a nearby surface.
Common parameters of the ablation process include (i) laser beam wavelength, (ii) laser pulse duration and (iii) laser beam fluence. Laser beam wavelength is an important factor because ablation requires sufficient absorption of the laser light into the material. Absorption wavelength characteristics are material-specific. Laser pulse duration is also an important parameter, as the mechanisms of ablation can vary substantially depending on the pulse length. Common pulse regimes include ultra-short (10 s of fsec-10 psec), very short (10 psec-1 nsec), short (1-200 nsec), long (1 μsec-1 msec) and continuous-wave (CW). Laser beam fluence refers to the measure of energy per unit area and is usually measured in J/cm2. The higher the fluence, the more “cutting ability” a laser has. This parameter is particularly important because the laser beam fluence must exceed the specific threshold fluence value, Fth, of the target material for the laser to actually dissolve the molecular bonds and remove material. Laser beam fluence below the Fth threshold value will increase a material's temperature, but will not melt or evaporate it. Threshold fluence values are material-specific, wavelength-specific and pulse duration-specific.
Laser ablation is thus greatly affected by the nature of the material and its ability to absorb energy, requiring that at the wavelength of the laser the material has sufficient absorption to enable ablation. The depth over which the laser energy is absorbed, and thus the amount of material removed by a single laser pulse, depends on the material's optical properties at the laser wavelength. Laser pulses can vary over a very wide range of durations (milliseconds to femtoseconds) and fluxes and can be precisely controlled.
Thus, laser ablation can be very valuable for both research and industrial applications. Laser ablation is often employed for precise material removal in the fabrication of advanced devices at the scale between microns and hundreds of microns and even at the scale of hundreds of centimeters, e.g., in case of solar panel fabrication. Both direct-write and mask-projection techniques are used, and laser wavelength is selected to be compatible with the materials being processed.
Common parameters of the laser drilling process include (i) laser wavelength, (ii) laser pulse energy, (iii) laser pulse duration, (iv) laser pulse repetition rate, (v) the number of laser pulses delivered, (vi) laser spot size and shape as delivered to the work-piece, (vii) laser energy density as delivered to the work-piece and (viii) the path and velocity of the scanning beam on the work-piece.
In laser machining, attaining high throughput for machining multiple parts corresponds to faster production, but more importantly, lower production cost, resulting in higher profits for a laser-machining organization. Thus, any improvement to throughput allows for a competitive advantage. This is true for either direct-write or mask projection machining.
Typically, to machine a three-dimensional object, either the object to be machined must be turned/rotated around one or more axes, the laser beam must be moved, and/or multiple laser beams must be used. This is so that all surfaces of the object requiring machining can be laser machined. In other words, either the work-piece or the output of the laser machining system must move (or be multiplexed in the case of multiple beams) so that all surfaces around the work-piece can receive laser treatment (e.g., 360 degrees).
In cases when the object to be machined must be turned/rotated around one or more axes, even with small objects, current systems utilize mechanical means. Such mechanical means are only capable of effecting the required movements with required precision and accuracy at limited speed, at the rate of approx. 0.3-3 sec per each movement. Often, additional time is required for settling vibrations caused by such motion. If the pattern to be machined consists of large number of features, the laser ablation process takes only a small fraction of the production time as compared to the time required for movement/motion of the object for machining for machining one feature at one location to another feature at another location, resulting in decreased efficiency (e.g., low duty cycle on laser usage). In addition, in situations where positioning registration by mechanical means does not provide adequate precision, other means such as machine vision alignment generally must be used. When multiple motions of the object are required, re-alignment after each motion adds time to the process cycle and further reduces the efficiency.
On the other hand, a galvanometer scanning approach enables the movement of a laser beam over comparable distances at the rate of 0.001-0.01 sec per each move, and times for settling of vibration for the galvanometer scanning head are significantly reduced as compared to the settling of vibrations due to rotation of the object for machining.